1. Field of the Invention
The present invention relates to metal nitride thin films. In particular, the present invention concerns a method of in situ reduction of source chemicals as well as a method of growing of metal nitride thin films. The present invention also relates to an apparatus for growing thin films on a substrate by an ALD type process.
2. Description of Related Art
The integration level of components in integrated circuits is increasing, which rapidly brings about a need for a decrease of the size of components and interconnects. Design rules are setting the feature sizes to xe2x89xa60.2 xcexcm. Deposition of uniform thin films on wafer surfaces by Physical Vapor Deposition (referred to as PVD hereinafter) and Chemical Vapor Deposition (referred to as CVD hereinafter) methods has become difficult due to small feature sizes. As a result, complete film coverage on deep bottoms of vias and trenches cannot be obtained. PVD methods require more or less direct line-of-sight on the surfaces to be coated Traditional CVD methods require rather precise concentration control of the source chemicals and good temperature uniformity over the substrate. Deep bottoms may have a local xe2x80x9cmicroclimatexe2x80x9d where the variable concentration of source chemical vapors is causing non-uniform growth of thin film.
Integrated circuits contain interconnects which are usually made of aluminium or copper. Especially copper is prone to diffusion to the surrounding materials. Diffusion affects the electrical properties of the circuits and active components may malfunction. The diffusion of metals from interconnects into the active parts of the devices is prevented with an electrically conductive diffusion barrier layer. Favoured diffusion barriers are, e.g., amorphous transition metal nitrides, such as TiN, TaN and WN. The nitrides can be non-stoichiometric because nitrogen is located at interstitial position of the lattice.
Atomic Layer Deposition (ALD), originally, Atomic Layer Epitaxy (ALE) is an advanced variation of CVD. The method name was changed from ALE into ALD to avoid possible confusion when discussing about polycrystalline and amorphous thin films. The ALD method is based on sequential self-saturated surface reactions. The method is described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811. The reactor benefits from the usage of inert carrier and purging gases which makes the system fast.
The separation of source chemicals from each other by inert gases prevents gas-phase reactions between gaseous reactants and enables self-saturated surface reactions leading to film growth which requires neither strict temperature control of the substrates nor precise dosage control of source chemicals. Surplus chemicals and reaction byproducts are always removed from the reaction chamber before the next reactive chemical pulse is introduced into the chamber. Undesired gaseous molecules are effectively expelled from the reaction chamber by keeping the gas flow speeds high with the help of an inert purging gas. The purging gas pushes the extra molecules towards the vacuum pump used for maintaining a suitable pressure in the reaction chamber. ALD provides an excellent and automatic self-control for the film growth.
In case of transition metal nitrides, reduction of the metal source material is needed in order to increase the amount of metal in the nitride and, thus, to lower the resistivity of the nitride. xe2x80x9cReductionxe2x80x9d can be defined as any reaction, wherein the metal of the source chemical receives electrons and its oxidation state decreases.
In the art, it is known to reduce the metal source material by pulsing a reducing agent after the metal source material pulse. A number of different chemicals have been used for the reduction. For example, tungsten compounds have been reduced by using hydrogen (H2) (U.S. Pat. No. 5,342,652 and EP-A2-899 779), silanes such as SiH4 (U.S. Pat. No. 5,691,235) and chlorosilanes such as SiHCl3 (U.S. Pat. No. 5,723,384). Low oxidation-state metal chlorides can also be synthesized by passing a gaseous mixture of hydrogen chloride (HCl) and hydrogen (H2) over a heated metal (U.S. Pat. No. 4,803,127).
Reduction of WF6 into W metal on substrate surfaces by using a silane, Si2H6, is disclosed by J. W. Klaus (Atomic Layer Deposition of Tungsten and Tungsten Nitride Using Sequential Surface Reactions, AVS 46th International Symposium, abstract TF-TuM6, http://www.vacuum.org/symposium/seattle/technical.html, to be presented on the 26th of October, 1999 in Seattle, USA).
There are, however, drawbacks related to these prior art methods. Silanes may also react with WF6, thus forming tungsten silicides, such as WSi2. Hydrogen can reduce a tungsten compound into tungsten metal which has too low vapor pressure for being transported in gas phase onto substrates.
Various metal species adsorbed on substrate surfaces have been reduced with zinc in ALE processes (cf., e.g., L. Hiltunen, M. Leskelxc3xa4, M. Mxc3xa4kelxc3xa4, L. Niinistxc3x6, E. Nykxc3xa4nen , P. Soininen, xe2x80x9cNitrides of Titanium, Niobium, Tantalum and Molybdenum Grown as Thin Films by the Atomic Layer Epitaxy Methodxe2x80x9d, Thin Solid Films, 166 (1988) 149-154). In the known processes, the additional zinc vapor used during the deposition decreased the resistivity of the nitride film either by increasing the metal-to-nitrogen ratio or by removing oxygen from the films. The known process comprised the following pulsing order: a metal source chemical vapor pulse/an inert gas purge/a zinc vapor pulse/an inert gas purge/a nitrogen source chemical vapor pulse/an inert gas purge. A basic problem related to reduction carried out with the zinc vapor method is that thin films contaminated with zinc metal and its compounds should be avoided in processes used for the manufacture of integrated circuits (referred to as IC hereinafter). Diffusing zinc can destroy the active components of the IC""s. Additionally, the low end of the substrate temperature range is probably limited by the volatility of zinc metal and the sticking coefficient of zinc compounds on the surface.
In addition to zinc, hydrogen and magnesium have also been tested as reducing agents in ALE processes. The results have not been promising, Hydrogen is not capable enough of reducing the metal compounds at low substrate temperatures. Magnesium forms on the substrate surface a halide which has a very low vapor pressure and stops film growth. It seems that the applicability of elemental reduction on the substrate surface is rather limited. Few elements have high enough vapor pressure to be used as ALD source chemicals. Even fewer gaseous elements form a volatile byproduct during the reduction step.
A method for influencing the properties of CVD source chemicals is disclosed by C.-Y. Lee [The Preparation of Titanium-Based Thin Film by CVD Using Titanium Chlorides as Precursors, Chem. Vap. Deposition, 5 (1999) 69-73)]. According to the publication, in a CVD process TiCl4 vapor was flowing over titanium metal which was heated to 900xc2x0 C. The reaction produced TiClx (x less than 4) subchlorides. These subchlorides were downstream thermally decomposed into Ti metal on a substrate which was heated to 500-800xc2x0 C.
The CVD reducing apparatus described by C.-Y. Lee can not be used in ALD because of the required performance and character of ALD source chemicals and the location of the reducing agent. If a titanium reducing agent is covered by titanium halide molecules and it is exposed to reactive nitrogen containing source chemical like ammonia a layer of very inert titanium nitride will grow on its surface. Thus the formed titanium nitride layer prevents the desired reduction reaction of TiCl4 gas pulses.
It is an object of the present process to eliminate the problems of prior art and to provide a novel method of reducing metal source materials in an ALD type process. It is another object of the invention to provide a novel method of preparing metal nitride thin films by an ALCVD type method
It is a further object of the present invention to provide an apparatus for growing metal nitride thin films by an ALD type method.
These and other objects together with the advantages thereof over known processes are accomplished as hereinafter described and claimed.
The present invention is based on the surprising finding that metal source compounds used in an ALD type process can be reduced at moderate temperatures before being adsorbed or chemisorbed on the surface of a substrate. According to the invention the metal source compounds are therefore conducted in gaseous state into a reduction zone in which they are contacted with a solid or liquid reducing agent maintained at an elevated temperature in order to produce a gaseous reaction product containing the metal species of the metal source compound at a lower state of oxidation. The reduced metal source material is then contacted with the substrate and deposited on it according to the principles of ALD.
In comparison to the modification method for CVD titanium source chemicals described by C.-Y. Lee above, the present in situ reduction is carried out at low temperatures in particular at temperatures close to the actual substrate temperature, whereas in the known CVD method, temperatures of 900xc2x0 C. were used. As the below examples show, the reduction of the metal source material in the reduction zone and the reaction between the metal source material and the metal species bound to and originating form the surface give rise to gaseous reaction products which easily can be removed from the reduction zone or from the reaction space and which have a sufficiently high vapor pressure for being used as source materials in an ALD process.
Taking titanium chloride as an example, the reduction TiCl4 at 400xc2x0 C. gives rise to titanium subchlorides, such as Ti2Cl6, which can be transported to the substrate by inert gas. The results are quite surprising, because it has generally been believed that titanium subhalides are not suitable for low-temperature ALD due to low vapor pressure.
The apparatus according to the present invention comprises a reduction zone placed in regard to the flow path of the gaseous reactants which are to be reduced, the reduction zone is placed before the substrate, such that gaseous reactants can be brought into contact with the reduction zone before they are subjected to surface reactions with the substrate. Typical positions are in the flow channel interconnecting the source material containers and the reaction chambers, and in the reaction chambers upstream from the substrates.
In the present invention the reduction zone is placed before the reaction zone. In the apparatus according to C.-Y. Lee [The Preparation of Titanium-Based Thin Film by CVD Using Titanium Chlorides as Precursors, Chem. Vap. Deposition, 5 (1999) 69-73)] the reducing zone and the substrate are located in the same space.
In ALD method the thermal decomposition of the source chemicals is prevented by applying low substrate temperature, thus thermal decomposition of low oxidation state titanium chlorides to metal state titanium on the substrate and to halogen molecules in the gas phase is prevented.
According to the present invention the reactivity and clean surface of the reducing agent is ensured by separating the reduction zone and the reaction zone from each other. Thus the metal source chemical pulses contacting the reducing agent always meet a surface capable of changing the oxidation state of the metal source chemicals.
More specifically, the method of reducing a metal source compound is characterized by what is stated in the characterizing part of claim 1.
The method of growing metal nitride thin films is characterized by what is stated in the characterizing part of claim 10.
The apparatus for growing metal thin films on a substrate is characterized by what is stated in the characterizing part of claim 18.
A number of considerable advantages are accomplished with the aid of the invention. The growing rate of the thin film is high, e.g., the growth rate of ALD titanium nitride thin film increased by almost a factor of 2 compared with the processes of the prior art. In addition the invention makes it possible to operate at low temperatures. When the reduction of the metal source material is carried out in situ, in other words, in the reactor system without using a separate reducing agent pulse, no additional reagent for reduction needs to be introduced into the reaction space. Thus, also one purging step is avoided. This leads to shorter cycle times and thus to more efficient growing of the films.
The reduction of the metal source material gives lower resistivity to metal nitride film, since the amount of metal increases with respect to the amount of nitrogen. The present process gives as good reduction properties and thus as good film resistivities as the processes of prior art with a simpler and faster growing procedure.
As mentioned above, the compounds formed as byproducts in the reduction reaction and in the reaction between the metal species on the surface of the substrate are essentially gaseous and they exit the reactor easily when purging with an inert gas. The amount of residues in the film is on a very low level.
A film grown with the present process exhibits good thin film properties. Thus, the metal nitride films obtained have an excellent conformality even on uneven surfaces and on trenches and vias. The method also provides an excellent and automatic self-control for the film growth.
The ALD grown metal nitride films can be used, for example, as ion diffusion barrier layers in integrated circuits. Tungsten nitride stops effectively oxygen and increases the stability of metal oxide capacitors. Transition metal nitrides and especially tungsten nitride is also suitable as an adhesion layer for a metal, as a thin film resistor, for stopping the migration of tin through via holes and improving the high-temperature processing of integrated circuits.
Next, the invention is described in detail with the aid of the following detailed description and by reference to the attached drawing.